1. Field of the Invention
The present invention relates generally to technology for assembling magnetic circuitry for use In a magnetic resonance imaging (MRI) system, and more specifically to a method of installing pole pieces at predetermined positions of permanent magnets.
2. Description of Related Art
Magnetic resonance imaging (MRI) is an imaging technique used primarily in medical settings or diagnoses to produce high quality images of the inside of the human body. MRI is based on the principles of nuclear magnetic resonance, a spectroscopic technique to obtain microscopic chemical and physical information about molecules,
As is known in the art, resistive electromagnets, permanent magnets, and super-conducting electromagnets have been used in the magnet field generators of MRI systems. The resistive electromagnets consist of many winding or coils of wire wrapped around a cylinder or bore through which an electric current is passed. Among these magnets, the super-conducting electromagnets are by far the most commonly used.
However, with the improvement of characteristics of permanent magnets using rare-earth elements, it is a current practice to use the permanent magnets in the MRI systems wherein the magnetic field strength is less than 0.5-Tesla (for example). Hereinafter, the permanent magnets made of rare-earth elements are referred to as magnets or permanent magnets.
In the MRI system using the permanent magnets, two magnets typically disk-shaped are installed in the system such as to face each other within a yoke structure. The two magnets respectively carry pole pieces on the major surfaces thereof (viz., at the predetermined positions of the magnets) such as to face each other in order to generate a uniform magnetic field in the space between the two magnets.
Each of the disk-shaped magnets, used in the MRI system, has usually a diameter of about 1(one) meter. Since it is practically impossible to fabricate the magnet with such a large diameter as a single unit, the magnet is manufactured by bringing together a plurality of magnetized blocks to assemble the permanent magnet Each magnetic block is fabricated by compressing magnetic power into a cube (for example) with each side ranging 4 to 10 cm, and thereafter sintering the cube and magnetizing the same. Such blocks, once magnetized, have extremely strong magnetic strengths, and the attracting force between the two magnets or between each magnet and a plate yoke on which the permanent magnets are assemble, reaches as strong as approximately 0.5-ton. Accordingly, in order to bring together the magnet blocks to form the disk-shaped magnets on the plain yoke, it is inevitable to prepare very stout and rigid assembling tools or structures. However, the manner of assembling the disk-shaped magnet using the blocks on the plate yoke is not directly concerned with the present invention, and accordingly, the description thereof will be omitted for the sake of simplifying the instant disclosure.
Before turning to the present invention, it is deemed preferable to briefly describe, with reference to FIGS. 1 and 2, the prior art relevant to the present invention, which is disclosed in the Japanese Laid-open Patent Application No. 2000-51175.
FIG. 1 is a diagram schematically showing a magnetic field generator 8 for use in an MRI system. As shown, the generator 8 comprises a pair of substantially rectangular plate yokes 10 and 12, which are rigidly coupled to four cylindrical column yokes 14a-14d, forming magnetic circuits or magnetic flux paths therewith. A disk-shaped magnet 18 is formed using a plurality of magnetic blocks 16, and being provided on the upper surface of the plate yoke 10, and carrying thereon a disk-like polo piece 19 which has a protrusion along the circumference thereof. As shown in FIG. 2, a magnet 17 is provided on the inner surface of the plate yoke 12 which carries a pole piece 21 which corresponds to the pole piece 19.
The magnetic field generator 8 shown in FIG. 1 is assembled as follows. Although not shown in the drawings, a plurality of magnetized blocks 16 are brought together to form the magnet 18 on the major surface of the plate yoke 10. Thereafter, the pole piece 19 is positioned on the magnet 18 against extremely strong magnetic forces therebetween. In more specific terms, when the pole piece 19 is brought into in the vicinity of the magnet 16, the magnetic attracting force reaches approximately 10-ton. Therefore, according to the prior art, the pole piece 19 is very slowly lowered using a crane with extreme care and attention. It is understood that such a crane should be very strong so as to resist the above-mentioned attracting forces of about 10-ton exerted on the pole piece 19.
Thereafter, the four column yokes 14a-14d are attached to the four corners of the plate yoke 10 as shown in FIG. 1. On the other hand, the other pole piece 21 (see FIG. 2) is positioned on permanent magnet 17 in the same manner as mentioned above while the magnet 17 faces upward. Subsequently, the plate yoke 12, which carries the magnet 17 and the pole piece 21 thereon, is upset so that the combined magnet 17 and pole piece 21 face downward. Following this, the plate yoke 12 is pulled upward using a large came and moved above the plate yoke 10 as best shown in FIG. 2. Then, the plate yoke 12 is carefully lowered inch-by-inch. A plurality of rods 20, which are received in corresponding holes 22 provided at the undersurface of the plate yoke 12, are to prevent the yoke plate 12 from undesirably fluctuating due to the strong magnetic field.
Thus, the prior art as mentioned above has encountered the problem that the installation of the pole pieces inevitably necessitates a stout or strong installing apparatus such as a strong crane, with the result that the settings of the pole pieces on the permanent magnets are expensive and time consuming to a considerable extent.